When converting a feedstock containing a hydrocarbon to a product in an industrial reactor, it is desirable to maximize the production of desired product or products, and to control, typically to minimize, the production of by-products. One type of reactor useful for conducting hydrocarbon conversion reactions is a fluidized bed reactor, wherein solid catalyst particles are suspended in a fluidized state during contact with the feedstock and other vapor materials. These types of reactors usually have a cylindrical reactor geometry. One method for reducing the production of by-products in a fluidized bed reactor involves operating in a hydrodynamic flow regime such that the superficial gas velocity obtains a velocity high enough to cause the net flow of catalyst in the reactor to flow in the same direction as the flow of the feedstock and other vapors, i.e., the feedstock and other vapors essentially carry the catalyst particles along with them. These flow regimes are known to those skilled in the art as the fast-fluidized bed and riser regimes, more generally as the transport regime, and are preferred in reaction systems in which a more plug flow reactor type is desired.
In general, for a given reactor cross sectional area (which in a cylindrical reactor geometry is proportional to the diameter, and more generally to a characteristic width or diameter), the catalyst concentration in a fluidized bed reactor decreases with increasing gas superficial velocity. Higher gas superficial velocities generally require taller reactor heights to allow a given amount of feedstock to contact a required amount of catalyst. These higher gas superficial velocities require a higher aspect ratio (the ratio of a reactor height to its diameter or characteristic width) of the reactor. Further, in many cases it is desirable to make a fluidized reactor with a very large cross sectional area to enable very large throughputs of feedstock in a single reactor facility. However, increasing fluid bed diameter, particularly in the transport regime, also requires increased reactor height. This increased height is needed because a certain minimum reactor height, in terms of a minimum aspect ratio, is required to achieve a fully developed flow pattern to approximate plug flow reactor behavior. At the exit and, particularly, at the entrance of a transport regime fluidized bed reactor, unsteady state momentum effects dominate hydrodynamic behavior (e.g., the energy required for the feedstock vapors to pick up and accelerate the solid catalyst against the force of gravity) in a manner not conducive to obtaining approximate plug flow behavior. Not until these momentum effects are dampened out by progressing along the reactor height will a well behaved, approximately plug flow fluid/solid flow pattern emerge. Finally, should the use of lower activity catalysts be desired in the transport regime, aspect ratios must also increase to provide desired higher feedstock conversion.
Unfortunately, high aspect ratio transport fluid bed reactors are difficult and expensive to construct and maintain. They are expensive because they must have a very large, heavy separation vessel at the top, often filled with heavy equipment, to capture and manage the flowing catalyst and reactor product. As the reactor increases in height (aspect ratio), more expensive support structures may be required. In certain areas of the world where inclement, especially windy weather occurs routinely, even more structural support is required, and certain aspect ratios are not economic. In these situations, a number of independent reactor systems with independent separation vessels may be required. With these multiple, complete and independent reactor systems come attendant multiplication of costs.
Thus, a need exists in the art for a reactor which can provide the desired aspect ratio without necessitating an unwieldy height, forcing a width in which the desired, fully developed flow regime may never be obtained, or without requiring multiple, independent reactor systems.